1. Field of the Invention
This invention relates generally to a pressure swing adsorption (PSA) unit for providing a purified gas and, more particularly, to a PSA unit for purifying hydrogen in a stand-alone fuel processor for a hydrogen fuel cell engine, where the PSA unit employs a pressure sensor for measuring the output pressure of the purified hydrogen and a mass flow controller for measuring and controlling the output pressure of the purified hydrogen to control the hydrogen purity.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular type of fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perflurosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. In some cases, the exhaust gas can be re-circulated so that any remaining oxygen therein can be used.
In vehicular fuel cell applications, it is desirable to use a liquid fuel, such as alcohols (methanol or ethanol), hydrocarbons (gasoline), and/or mixtures thereof, such as blends of ethanol/methanol and gasoline, as a source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store on the vehicle. Further, there is a nationwide infrastructure for supplying liquid fuels. Gaseous hydrocarbons, such as methane, propane, natural gas, LPG, etc., are also suitable fuels for both vehicle and non-vehicle fuel cell applications.
Hydrocarbon-based fuels must be disassociated to release the hydrogen therefrom for fueling the cell. The disassociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors where the fuel reacts with steam, and sometimes air, to generate a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in steam methanol reformation process, methanol and water are reacted to generate hydrogen and carbon dioxide. However, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor that contains two sections. One section is primarily a partial oxidation reactor (POX) and the other section is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water.
The known fuel processors also typically include downstream reactors, such as a water/gas shift (WGS) reactor and a preferential oxidation (PROX) reactor. The PROX reactor is necessary to remove carbon monoxide in the reformate gas because carbon monoxide contaminates the catalytic particles in the PEM fuel cell. The PROX reactor selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2) using oxygen from air as an oxidant. However, the use of a PROX reactor in a fuel processor affects processor performance. For example, control of the air feed is important to selectively oxidize CO to CO2. Also, the PROX reactor is not 100% selective, and thus results in consumption of hydrogen. Therefore, some hydrogen that would normally be available to provide power is consumed by the PROX reactor. Hence, less power output is provided per a given size stack of fuel cells. Further, the heat generated from the PROX reactor is at low temperature, resulting in excess low-grade heat. Also, typical catalysts used in a PROX reactor contain precious metals, such as platinum or iridium, which are very expensive.
The hydrogen generated in a fuel processor using a PROX reactor for CO clean-up typically contains less than 50% hydrogen, where the balance of the hydrogen-rich reformate gas consists primarily of carbon dioxide, nitrogen and water. Thus, the reformate gas is not suitable for compression and storage because much energy would be wasted in compressing the non-hydrogen components in the reformate gas. Also, valuable storage space would be wasted to contain the non-hydrogen components.
Certain techniques do exist in the art for generating nearly pure hydrogen in non-automotive fuel processing systems. One technique of generating pure hydrogen in a fuel processing system includes the use of hydrogen permeable membranes. These membranes selectively allow the hydrogen to pass through and prevent the other by-products in the reformate gas from permeating through. Typical membranes for these applications contain palladium, which is very expensive. Also, these membranes only operate at relatively high temperatures (250–550° C.), and thus, it takes a long time after the low temperature start-up for a fuel processing system containing hydrogen permeable membranes to be able to generate hydrogen. Additionally, these membranes operate at very high pressures (>5 bar), which leads to high compressor loads and inefficient systems.
It has been suggested in the art that a pressure swing adsorber (PSA) unit can be used to generate nearly pure hydrogen from the reformate gas in a fuel processor system. A fuel cell system employing a PSA unit is described in commonly owned U.S. patent application Ser. No. 09/780,184, published Aug. 15, 2002 as publication No. US 2002/0110504 A1, and herein incorporated by reference. In the fuel cell system disclosed in the '184 application, the PSA unit is integrated within the fuel cell stack. The PSA unit uses the anode off-gas from the fuel cell as a purge stream within the PSA unit or uses the cathode off-gas from the fuel cell to combust the low-pressure exhaust gas from the PSA unit. Additionally, both the anode and cathode off-gas can be used. Such a system could not be used as a stand-alone hydrogen generator, where the hydrogen gas is stored for subsequent use in a fuel cell engine.
U.S. patent application Ser. No. 10/389,375, filed Mar. 13, 2003, titled “Fuel Processor Module for Hydrogen Production for a Fuel Cell Engine Using Pressure Swing Adsorption,” assigned to the assignee of this application, and herein incorporated by reference, also discloses another fuel processor system employing a pressure swing adsorption unit.
In one design, the PSA unit is a rapid-cycle device that includes one input port and two output ports. The reformate gas being purified enters the PSA unit through the input port, the purified hydrogen gas exits the PSA unit through one of the output ports and an exhaust gas including the non-hydrogen gases in the reformate gas exits the PSA unit through the other output port. The PSA unit includes a plurality of compartments or vessels that include an adsorbent that adsorbs the non-hydrogen by-products in the reformate gas. The vessels are cycled between high pressure and low pressure states. When a particular vessel is in a high pressure state, the adsorbent adsorbs the by-products, but the smaller hydrogen atoms do not get adsorbed.
PSA units are typically very large and consist of a minimum of two separate adsorption vessels including numerous valves and manifolds. In a two-vessel system, one vessel would be in the adsorption mode and the other vessel would be in various stations of blow-down, purge and pressurization. Many commercial hydrogen PSA units use four vessels, where one vessel is in the adsorption mode at any given time, and the other three vessels are in the various stages of equalization, blow-down, purge and pressurization. Also, some commercial hydrogen PSA units employ twelve vessels, with four vessels in the adsorption mode at any given time, and the other eight vessels in the various stages of equalization, blow-down, purge and pressurization. It is well known that PSA units with more than two vessels exhibit higher hydrogen recoveries and reduced power by incorporating pressure equalization steps. These PSA units, however, include complex valve arrangements and are non-continuous due to the cycling of these valves.
One known way to cycle a multi-bed PSA unit is with rotary valves. PSA systems employing rotary valves are described in U.S. Pat. Nos. 4,925,464; 5,112,367 and 5,366,541. These patents describe devices that use a single rotary valve that rotates relative to a stationary port plate to direct gases to the various vessels in the PSA system as defined by the PSA cycle. U.S. patent application Ser. No. 10/706,320, filed Nov. 12, 2003, titled “Hydrogen Purification Process Using Pressure Swing Adsorption for Fuel Cell Applications,” assigned to the assignee of this application, and herein incorporated by reference, discloses a PSA unit employing two rotary valves and defines a particular PSA cycle.
Systems with only one rotary valve directing flow to the feed end of adsorbent vessels are limited to using PSA cycles with feed-feed equalization. However, any cycle that can be defined using a collection of valves can be replicated by using rotary valves, one at the feed of the adsorbent vessels and one at the product end of the vessels. Such PSA systems with two rotary valves are described in U.S. Pat. Nos. 5,820,656 and 5,891,217.
A challenge in operating PSA systems that employ rotary valves is the ability to properly control the speed of the rotary valves with changes in the demand of the product gas. It is known that PSA units are employed in the medical industry to provide purified oxygen for hospital uses. For oxygen purifying PSA units, an oxygen purity sensor is employed to control the speed of the rotary valve within the unit to ensure that the purified gas output has the desired oxygen purity.
A hydrogen purity sensor could be employed in a hydrogen purifying PSA unit to ensure that the product purity is maintained at the purified gas output. However, such hydrogen purity sensors are typically very costly and bulky, and thus do not have significant applicability for an automotive application. Further, such hydrogen purity sensors have a limited range of measurement and cannot provide real-time data measurement.